mm: Introduce mm_struct.has_pinned

(Commit message majorly collected from Jason Gunthorpe)

Reduce the chance of false positive from page_maybe_dma_pinned() by
keeping track if the mm_struct has ever been used with pin_user_pages().
This allows cases that might drive up the page ref_count to avoid any
penalty from handling dma_pinned pages.

Future work is planned, to provide a more sophisticated solution, likely
to turn it into a real counter.  For now, make it atomic_t but use it as
a boolean for simplicity.

Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: xzpeter

include/linux/mm_types.h

@@ -436,6 +436,16 @@ struct mm_struct {
 		 */
 		atomic_t mm_count;
 
+		/**
+		 * @has_pinned: Whether this mm has pinned any pages.  This can
+		 * be either replaced in the future by @pinned_vm when it
+		 * becomes stable, or grow into a counter on its own. We're
+		 * aggresive on this bit now - even if the pinned pages were
+		 * unpinned later on, we'll still keep this bit set for the
+		 * lifecycle of this mm just for simplicity.
+		 */
+		atomic_t has_pinned;
+
 #ifdef CONFIG_MMU
 		atomic_long_t pgtables_bytes;	/* PTE page table pages */
 #endif

kernel/fork.c

@@ -1011,6 +1011,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	mm_pgtables_bytes_init(mm);
 	mm->map_count = 0;
 	mm->locked_vm = 0;
+	atomic_set(&mm->has_pinned, 0);
 	atomic64_set(&mm->pinned_vm, 0);
 	memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
 	spin_lock_init(&mm->page_table_lock);

mm/gup.c

@@ -1255,6 +1255,9 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
 		BUG_ON(*locked != 1);
 	}
 
+	if (flags & FOLL_PIN)
+		atomic_set(&current->mm->has_pinned, 1);
+
 	/*
 	 * FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
 	 * is to set FOLL_GET if the caller wants pages[] filled in (but has
@@ -2660,6 +2663,9 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 				       FOLL_FAST_ONLY)))
 		return -EINVAL;
 
+	if (gup_flags & FOLL_PIN)
+		atomic_set(&current->mm->has_pinned, 1);
+
 	if (!(gup_flags & FOLL_FAST_ONLY))
 		might_lock_read(&current->mm->mmap_lock);